This invention relates to an optical shaft angle encoder which produces electrical signals indicative of the angular position of a shaft. In particular, it relates to a symmetrical arrangement of photodetectors, and windows and spokes in a code wheel for reliably producing an index pulse to indicate one angular position of the shaft.
Incremental shaft angle encoders are used to resolve the position and movement of a code wheel which rotates with a shaft. Such encoders include a light source for emitting a light beam, an array of transparent windows and opaque spokes on the code wheel for modulating the light beam in response to rotation of the wheel, and a detector assembly for receiving modulated light in producing electrical signals indicating the amount of light received by the photodetectors.
For a shaft angle encoder to produce an indication of the absolute position or location of the code wheel as it rotates, an index pulse is generated at least once per revolution of the code wheel. The incremental signals from a data track on the code wheel are used to count incremental movement from the index pulse for determining the absolute angular position of the code wheel at any place along its rotation. Thus, such a shaft angle encoder includes at least two channels of information. One of the information channels comprises an index signal produced at least once during a revolution at a known angular position and a data signal indicating incremental changes from the known position.
Shaft angle encoders are described, for example, in U.S. Pat. Nos. 4,259,570, 4,266,125, 4,451,731, and 4,691,101, the subject matter of which is hereby incorporated by reference.
A high degree of resolution and accuracy may be needed in such a shaft angle encoder. It is not unusual to specify a resolution of 2000 increments per revolution of the shaft. To achieve such a resolution, the windows through the code wheel through which light passes in the area of the photodetectors must be narrow. As a consequence, the amount of illumination and signal strength from the photodetectors tends to be low. To achieve a suitable signal strength, two or three windows and corresponding photodetectors may be used for each track to obtain adequate signal strength while keeping the index pulse width narrow.
In an exemplary embodiment, the code wheel has 500 transparent windows and an equal number of opaque spokes between the windows in the data track. These windows and spokes have a trapezoidal shape since they are located immediately adjacent to one another on the circular track. An exemplary nominal width of each window and spoke is 62 microns and the radial length of each window is about 750 microns. The code wheel is made of an optically opaque material such as stainless steel and has a diameter of approximately 22 millimeters. The windows are holes masked and etched through the disk. The dimensions of the windows and photodetectors for obtaining an index pulse are comparable. Such numerical values are merely exemplary.
With such very small dimensions, careful alignment of the parts of the shaft angle encoder becomes of appreciable importance. As a way of alleviating some of the alignment stringency, a technique has been developed for obtaining index pulses using two tracks. In such an arrangement, one index track is radially inward of the data track and the other is radially outward from the data track. Thus, the centroid of the two index tracks is at the same radial distance from the shaft as the data track.
Photodetectors are arranged for these index tracks so that when one set of photodetectors is illuminated when the code wheel is in its index position, the other set of photodetectors is occulted.
A push-pull electronic arrangement is then used for determining the location of the index pulse. In such an arrangement, when one of the tracks, such as the outer track, has greater illumination of the photodetectors, there is a logic signal of one sense, such as a logical 1. When the illumination on the inner track becomes greater than that of the output track, there is the opposite sense of logic signal, such as a logical 0, which forms the index pulse.
The windows and photodetectors are arranged so that in only one position does the illumination of the inner track exceed the illumination of the outer track. This is accomplished by having the photodetectors for the inner track aligned with windows in that position, and the photodetectors for the outer track being aligned with opaque spokes.
In previous arrangements for generating such index pulses where there are multiple windows for greater signal strength, the windows have been located asymmetrically in a radial or circumferential direction for minimizing what are referred to as side lobes relative to the index pulse.
Assume there are two windows in a track. When both windows are over photodetectors there is a photodetector current with a magnitude of two. As the code wheel continues to rotate, one of the windows passes over a second photodetector while the other photodetector remains dark. This produces a side lobe photocurrent signal having a magnitude of one. The asymmetrical arrangement of windows extending around an index track tends to minimize the magnitude of the side lobe signals relative to the index signal.
Some code wheels have been made with symmetrical index tracks, but the index tracks are offset in a circumferential direction from symmetry with the data track. This has been done so that phase differences may be used for determining the direction of rotation of the code wheel.
Situations have arisen where it is desirable to have a completely symmetrical code wheel. This occurs, for example, where it is desirable to switch the photodetectors and light source to opposite faces of the wheel; that is, turn the wheel over. When an asymmetrical code wheel is used, appreciable care must be taken to assure that the wheel orientation is correct. Incorrect assembly with the wheel upside down can make proper operation of the encoder impossible.
It is desirable to have a completely symmetrical code wheel so that this potential error in assembly can be avoided and assembly operations thereby expedited. A symmetrical code wheel also eliminates the occasional errors that may occur during the best of assembly operations, either during original manufacture or in subsequent reassembly.